Studies of humoral and cell-mediated immune responses induced by EBV are being carried out in order to identify persons with impaired immunological recovery manifested by an EBV-VCA antibody titer of greater than or equal to 1:320. Once identified, the high risk persons will be studied further for humoral and cell-mediated responses. Quantitative titrations in 72 IM cases and 72 controls have been collected and frozen in DMSO from these patients for later tests of CMI to EBV and other antigens. Additional studies on the origin of heterophile antibody in these cases indicate that horse red cells form rosettes with lymphocytes during acute infectious mononucleosis but not from normal controls. Further studies of 17 lymphocyte preparations from 11 acute IM patients have confirmed this with horse RBC rosettes varying from 4 to 51 percent. However, studies of lymphocytes from 12 normal persons indicate that 0 to 35 percent of such lymphocytes are also rosetted by horse rbc's. Various factors, such as fetal calf serum (a source of heterophile antigen) are under study to determine the source of result variability.